In recent years, there is a tendency to increase the combustion pressure in vehicle engines for high power output and fuel efficiency improvement. Consequently, the voltage required of spark plugs of the engines at ignition tends to become high. The rate of wear of electrodes of the spark plugs increases with increase in the voltage required of the spark plugs at the ignition. It has thus been demanded to develop techniques for suppressing wear of the electrodes of the spark plugs.
Conventionally known is a technique to provide noble metal tips on opposed surfaces of the center and ground electrodes of the spark plug for suppression of wear of the electrodes of the spark plugs (see, for example, Japanese Laid-Open Patent Publication No. 2008-77838).
Depending on the voltage required of the spark plug, however, there may occur melting of the noble metal tips themselves. There has accordingly been a demand for a technique to suppress wear of the electrodes irrespective of the materials of the electrodes.